A spark plug used for internal combustion engines is mounted on an internal combustion engine so as to ignite an air-fuel mixture. Generally, a spark plug is composed of an insulator having an axial bore, a center electrode inserted in the axial bore, a metal shell disposed on an outer circumference of the insulator and a ground electrode provided at a front end face of the metal shell and forming a spark discharge gap with the center electrode. Generally, when the metal shell and the insulator are assembled, a metal shell taper portion provided on an inner circumferential face of the metal shell and an insulator taper portion provided on an outer circumferential face of the insulator are fixed together through a metal plate packing.
In a combustion chamber, carbon is produced due to an incomplete combustion of air-fuel mixture or the like. The carbon is accumulated on a surface of a part of the insulator (an insulator nose) that is exposed to the air-fuel mixture or combustion gas. When a certain amount of carbon is accumulated and covers on the surface of the insulator nose, electric current leaks from the center electrode to the metal shell through the carbon deposited on the insulator nose, whereby a normal spark discharge in the spark discharge gap tends to be interrupted.
In order to prevent this problem, it is known to extend the insulator nose of the insulator. In this way, even though a certain amount of carbon is accumulated, the surface of the insulator nose in unlikely to be covered with carbon, thereby enhancing an anti-fouling performance of the spark plug.
However, when the insulator nose is extended, heat is not smoothly transferred from the insulator to the metal shell because the length of a portion of the insulator that is adjacent to the metal shell and that is disposed at a front end side with respect to the plate packing is necessarily reduced. Thus, heat conduction of the insulator is likely to be deteriorated.
Therefore, it is disclosed that the diameter of the front end portion of the insulator is reduced in two levels (i.e., so-called a “double tapered shape”) so that an outer circumferential face of a portion between a first step taper portion and a second step taper portion can be close to an inner circumferential face of the metal shell taper portion. See Japanese Patent Application Laid-Open (kokai) No. 2005-183177. Thus, heat can be smoothly transferred from the insulator to the metal shell. As a result, heat conduction of the insulator can be improved, whereby the insulator nose can be further extended.
However, even though the insulator nose is extended, an improvement in anti-fouling performance is not fully achievable compared to a conventional spark plug. In this respect, as shown in FIG. 5, when a region from a front end of the insulator 51 to a location “J”, which defines a gap “g” having an equal dimension to that of a spark discharge gap “G” with an inner circumferential face of a metal shell 52, is covered with carbon, a discharge (flashover) tends to occur from the location J into the metal shell 52 due to the carbon. Particularly, when a distance “H” between the front end of the insulator 51 and the location J along an axial C1 is reduced, the front end of the insulator 51 and the location J tends to be covered with carbon, causing flashover. As a result, a normal spark discharge in the spark discharge gap is possibly interrupted.
The present invention addresses the above-described problems. An advantage of the present invention is a spark plug for use in an internal combustion engine that is capable of improving heat conduction, as well as dramatically improving anti-fouling performance of the spark plug.